Improves evaluation of group-by expression

I was unable to group-by nested properties eg. `group-by="country.name"`or functions. This PR uses Angular's `$parse` method to resolve the expression and seems to work better.

I have had to leave the `$scope.$eval` call in for compatibility – without it you need to write the expression without quotes `group-by="country.name"` rather than `group-by="'country.name'"` which I would prefer but breaks the tests and therefore existing implementations.